The present invention relates to a power controlling apparatus for transferring a boiling water atomic reactor to a low power stand-by operation when a trouble to lower the power of the atomic reactor, e.g., a load rejection, a turbine trip or the like occurs in or out of a boiling water atomic power plant.
When the load of a generator is rejected due to a trouble of a power transmission system, for example, in a boiling water atomic power generating facility, a main steam control valve is closed to limit the supply of the steam to a turbine so that the overrun of the turbine is prevented, a turbine bypass valve is opened to release excessive steam to a condenser, and a scram signal is outputted to scram the atomic reactor. The trouble in the power transmission system can be frequently repaired in a relatively short time. If the atomic reactor is, however, once scrammed, complicated procedure is necessary to restart the operation of the atomic reactor thereafter, and it takes a plenty of time. Accordingly, the trouble causes drawbacks and disadvantages of lowering the operating efficiency of the atomic power plant. In order to heretofore prevent such drawbacks and disadvantages, the atomic reactor is not scrammed, when a trouble to lower the power of a generator, e.g., the rejection of the load of the generator, or the like occurs, but is transferred to the stand-by operation with low power so as to readily and rapidly restart the normal power operation of the atomic reactor immediately after the trouble to lower the power is removed, thereby improving the operating efficiency of the atomic power plant. In order to transfer the atomic reactor to the low power stand-by operation, a recirculation pump for the atomic reactor is first transferred to the minimual flow rate operation to reduce the core flow rate, a turbine bypass valve is simultaneously opened to release excessive steam to a condenser. Accordingly, the void fraction in the core is abruptly increased, and the power of the atomic reactor is thus lowered to a low power operation. In as much as a boiling water atomic power plant usually employs extracted steam from the turbine as a heat source for heating feed water, if the turbine bypass valve is opened and the atomic reactor is thus transferred to the low power operation, the extracted steam cannot be sufficiently obtained from the turbine, resulting in that the feed water heating capacity will decrease or be lost. Thus, the temperature of the feed water is lowered to decrease the void friction in the reactor core and accordingly the thermal output of the core which is once lowered due to the transfer of the recirculation pump to the minimum flow rate operation will increase, as shown by a curve A in FIG. 1, as the time is elapsed, and will hence reach higher power level than the reference level capable of maintaining stable power with low core flow rate with the result that the core stability is disadvantageously poor, or the atomic reactor reaches the scram level in the low flow rate range, causing the scram of the reactor.
For that purpose, parts of control rods selected in advance, (selected control rods), are quickly inserted, when the recirculation pump for the atomic reactor is transferred to the minimum flow rate operation.
Thus, the increase of the thermal power of the core can be suppressed, as shown by curves B1 to B4 in FIG. 1, by insertion of the selected control rods to maintain the atomic reactor in low power stand-by operation state. In this case, the selected control rods are selected which are arranged in a pattern which is most preferred to restart the normal power operation and to raise the power thereafter. Conventionally, when a trouble to lower the power of the atomic reactor, e.g., the load rejection of the generator or the like heretofore occurred, the recirculation pump for the atomic reactor was automatically transferred to the minimum flow rate operation and all the selected control rods where simultaneously abruptly inserted. When the atomic reactor is thus transferred to the low power stand-by operation, the power of the atomic reactor is feasibly affected by an operating state before the load of the generator is rejected. If the atomic reactor operated in high power state is, for example, transferred, as described above, to the partial power operation, its settled power becomes relatively high. If the atomic reactor operated in low power state is transferred under the same condition as above to the low power standby operation, its settled power becomes relatively low. FIG. 1 shows examples of calculated results. The curves B.sub.0 to B.sub.3 represent the thermal power variations of core with time when load is rejected in the initial conditions of 100, 80 and 60% rated operation of the thermal output at the 100% rated flow rate and the selected control rods of the same amount are inserted.
When the load of the generator is heretofore rejected and the atomic reactor is thus transferred to the low power stand-by operation in this manner, it was difficult to transfer the power of the atomic reactor to a desired or predetermined power level, and if the thermal power of the atomic reactor transferred to the low power stand-by operation is high, its stability is poor. Since the necessary number of partially inserted control rods delicately vary according to the state of core, it is also difficult to countermeasure against this variation in advance.
When an atomic power generating system is operated with low power to gain a source for station house load necessary for an isolated grid operation, it is preferred to set the output power of generator to approximately 10% of the rated output power. If the selected control rods are set so as to maintain stably the low power stand-by operation even when a load rejection occurs in case where the atomic reactor is normally operated at the highest level, the power of the atomic reactor is excessively lowered after the atomic reactor is transferred to the low power stand-by operation in case where such a trouble occurs when the atomic reactor is normally operated at relatively low power. This might cause the failure of obtaining the output power of the generator for a isolated grid operation. When a cause of load rejection is removed, the atomic reactor power is increased to restart a normal operation of atomic reactor. Various control rod operations are so effected as to prevent a local power increase of the core and to secure the quality of fuel during the increase in the reactor power. If the power of the atomic reactor is excessively lowered in this case as described above, the operations of the control rods during the increase of the power of the atomic reactor becomes frequent and simultaneously the operation state will enter into a control range of the control rod operation sequence controller provided for safety. Thus, the operations of control rods are restricted. This takes a long time to transfer the reactor to its normal operation as its drawbacks.